Guards may be included in defuns as usual but are ignored from the
perspective of admission to the logic: functions must terminate on
all arguments.

As in Nqthm, primitive functions, e.g., + and car, logically
default unexpected arguments to convenient values. Thus, (+ 'abc 3)
is 3 and (car 'abc) is nil. See programming, and see
the documentation for the individual primitive functions.

In contrast to earlier versions of ACL2, Version 1.8 logical
functions are executed at Nqthm speeds even when guards have not
been verified. In versions before 1.8, such functions were
interpreted by ACL2.

Colors have been eliminated. Two ``defun-modes'' are supported,
:program and :logic. Roughly speaking, :program does what :red used
to do, namely, allow you to prototype functions for execution
without any proof burdens. :Logic mode does what :blue used to do,
namely, allow you to add a new definitional axiom to the logic. A
global default-defun-mode is comparable to the old default color.
The system comes up in :logic mode. To change the global
defun-mode, type :program or :logic at the top-level. To specify
the defun-mode of a defun locally use

Guards can be seen as having either of two roles: (a) they are a
specification device allowing you to characterize the kinds of
inputs a function ``should'' have, or (b) they are an efficiency
device allowing logically defined functions to be executed directly
in Common Lisp. If a guard is specified, as with xargs:guard, then
it is ``verified'' at defun-time (unless you also specify xargs:verify-guards nil). Guard verification means what it always has:
the input guard is shown to imply the guards on all subroutines in
the body. If the guards of a function are verified, then a call of
the function on inputs satisfying the guard can be computed directly
by Common Lisp. Thus, verifying the guards on your functions will
allow them to execute more efficiently. But it does not affect
their logical behavior and since you will automatically get Nqthm
speeds on unverified logical definitions, most users will probably
use guards either as a specification device or only use them when
execution efficiency is extremely important.

Given the presence of guards in the system, two issues are unavoidable.
Are guards verified as part of the defun process? And are guards checked
when terms are evaluated? We answer both of those questions below.

We now turn to the evaluation of expressions. Even if your functions contain
no guards, the primitive functions do and hence you have the choice: when you
submit an expression for evaluation do you mean for guards to be checked at
runtime or not? Put another way, do you mean for the expression to be
evaluated in Common Lisp (if possible) or in the logic? Note: If Common Lisp
delivers an answer, it will be the same as in the logic, but it might be
erroneous to execute the form in Common Lisp. For example, should
(car 'abc) cause a guard violation error or return nil?

The top-level ACL2 loop has a variable which controls which sense of
execution is provided. To turn ``guard checking on,'' by which we
mean that guards are checked at runtime, execute the top-level form
:set-guard-checking t. To turn it off, do :set-guard-checking nil.
The status of this variable is reflected in the prompt.

means guard checking is off. The exclamation mark can be thought of
as ``barring'' certain computations. The absence of the mark
suggests the absence of error messages or unbarred access to the
logical axioms. Thus, for example

ACL2 !>(car 'abc)

will signal an error, while

ACL2 >(car 'abc)

will return nil.

Note that whether or not guards are checked at runtime is
independent of whether you are operating in :program mode or
:logic mode and whether theorems are being proved or not.
(Although it must be added that functions defined in :program
mode cannot help but check their guards because no logical
definition exists.)

Version 1.8 permits the verification of the guards of theorems, thus
insuring that all instances of the theorem will evaluate without
error in Common Lisp. To verify the guards of a theorem named
name execute the event

(verify-guards name).

If a theorem's guards have been verified, the theorem is guaranteed
to evaluate without error to non-nil in Common Lisp (provided
resource errors do not arise).

Caveat about verify-guards: implies is a function symbol, so in the
term (implies p q), p cannot be assumed true when q is evaluated;
they are both evaluated ``outside.'' Hence, you cannot generally
verify the guards on a theorem if implies is used to state the
hypotheses. Use if instead. In a future version of ACL2, implies
will likely be a macro.

See sum-list-example.lisp for a nice example of the use of Version
1.8. This is roughly the same as the documentation for
guard-example.

We have removed the capability to do ``old-style-forcing'' as
existed before Version 1.5. See note5.

NOTE: Some low level details have, of course, changed. One such
change is that there are no longer two distinct type prescriptions
stored when a function is admitted with its guards verified. So for
example, the type prescription rune for binary-append is now

Nqthm-style forcing on linear arithmetic assumptions is no longer
executed when forcing is disabled.

Functional instantiation now benefits from a trick also used in
Nqthm: once a constraint generated by a :functional-instance
lemma instance (see lemma-instance) has been proved on behalf
of a successful event, it will not have to be re-proved on behalf of
a later event.

1+ and 1- are now macros in the logic, not functions. Hence, for
example, it is ``safe'' to use them on left-hand sides of rewrite
rules, without invoking the common warning about the presence of
nonrecursive function symbols.

The symbols force, mv-nth, and acl2-count have been added
to the list *acl2-exports*.

We now permit most names from the main Lisp package to be used as
names, except for names that define functions, macros, or constants.
See name.

We have changed the list of imports from the Common Lisp package to
ACL2, i.e., the list *common-lisp-symbols-from-main-lisp-package*,
to be exactly those external symbols of the Common Lisp package as
specified by the draft Common Lisp standard. In order to
accommodate this change, we have renamed some ACL2 functions as
shown below, but these and other ramifications of this change should
be transparent to most ACL2 users.

warning --> warning$
print-object --> print-object$

Proof trees are no longer enabled by default. To start them up,
:start-proof-tree.

We have added the capability of building smaller images. The
easiest way to do this on a Unix (trademark of AT&T) system is:
make small.

We fixed a bug in the hint mechanism that applied :by, :cases, and
:usehints to the first induction goal when the prover reverted to
proving the original goal by induction.

We fixed a bug in the handling of (set-irrelevant-formals-ok :warn).

In support of removing the old-style forcing capability, we deleted
the initialization of state global old-style-forcing and deleted the
definitions of recover-assumptions, recover-assumptions-from-goal,
remove-assumptions1, remove-assumptions, and split-on-assumptions,
and we renamed split-on-assumptions1 to split-on-assumptions.

The special value 'none in the proof-checker commands claim and =
has been replaced by :none.

A bug in the handling of hints by subgoals has been fixed. For
example, formerly a :do-not hint could be ``erased'' by a :use hint
on a subgoal. Thanks go to Art Flatau for noticing the bug.

The functions weak-termp and weak-term-listp have been
deleted, and their calls have been replaced by corresponding calls
of pseudo-termp and pseudo-term-listp. The notion of
pseudo-termp has been slightly strenthened by requiring that
terms of the form (quote ...) have length 2.

Performance has been improved in various ways. At the prover level,
backchaining through the recognizer alist has been eliminated in
order to significantly speed up ACL2's rewriter. Among the other
prover changes (of which there are several, all technical): we no
longer clausify the input term when a proof is interrupted in favor
of inducting on the input term. At the IO level, we have improved
performance somewhat by suitable declarations and proclamations.
These include technical modifications to the macros mv and
mv-let, and introduction of a macro the-mv analogous to the
macro the but for forms returning multiple values.

The proof-checker command expand has been fixed so that it
eliminates let (lambda) expressions when one would expect it to.

A new primitive function, mv-nth, has been introduced. Mv-nth
is equivalent to nth and is used in place of nth in the
translation of mv-let expressions. This allows the user to
control the simplification of mv-let expressions without
affecting how nth is treated. In that spirit, the rewriter has
been modified so that certain mv-nth expressions, namely those
produced in the translation of (mv-let (a b c)(mv x y z) p), are
given special treatment.

A minor bug in untranslate has been fixed, which for example will
fix the printing of conjunctions.

Translate now takes a logicp argument, which indicates whether it
enforces the restriction that :program mode functions do not occur
in the result.

The modified version of trace provided by ACL2, for use in raw Lisp,
has been modified so that the lisp special variable *trace-alist*
has a slightly different functionality. This alist associates,
using eq, symbols with the print representations of their values.
For example, initially *trace-alist* is a one-element list
containing the pair (cons 'state '|*the-live-state*|). Thus, one
may cons the pair (cons '*foo* "It's a FOO!") on to *trace-alist*;
then until *foo* is defined, this change will have no effect, but
after for example

(defconst *foo* 17)

then trace will print 17 as "It's a FOO!".

Trace also traces the corresponding logic function.

Proof-tree display has been improved slightly in the case of
successful proofs and certain event failures.